Requirement to get BAR pci_bus_address in user space

[hch@lst.de (Christoph Hellwig)]

On Thu, Jun 14, 2018@08:50:15AM -0600, Alex Williamson wrote:
> I don't understand the CQ vs CMB, but I think I gather that there's some
> sort of buffer that's allocated from within the devices MMIO BAR and
> some programming of the device needs to reference that buffer.
> Wouldn't you therefore use the vfio type1 IOMMU MAP_DMA ioctl to map
> the BAR into the IOVA address space and you can then use the IOVA +
> offset into the BAR for the device to reference the buffer?  It seems
> this is the same way we'd setup a peer-to-peer mapping, but we're using
> it for the device to reference itself effectively.  Thanks,

That's exactly what I meant..